Apparatus and method for generating ice pellets

ABSTRACT

An apparatus and method for generating ice pellets, in particular for controlling the properties of the generated ice pellets. The apparatus comprises a pellet generation region; at least one nozzle configured to supply a plurality of water droplets to the pellet generation region; a liquefied gas supply configured to deliver a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets; at least one temperature measuring device configured to obtain data indicative of the temperature of the ice pellets at generation; and a control system configured to adjust the flow rate of water and/or liquefied gas to thereby control the temperature of the ice pellets at generation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This specification is based upon and claims the benefit of priority from United Kingdom patent application number GB 1902893.5 filed on Mar. 4 2019, the entire contents of which are incorporated herein by reference.

BACKGROUND Field of the Disclosure

The present disclosure relates to an apparatus and method for generating ice pellets, in particular an apparatus and method which allows for improved control of the physical properties of the pellets that are generated.

Description of the Related Art

In applications where cleaning of surfaces of abrasion of coatings is required, shot blasting is typically used in many applications. For example, these techniques may be used to remove a cracked or poorly adhered coating from a target, to “blend” the edges of an area in order to smooth the boundary between a component and a coating, to provide a level of roughness to allow adhesion with a new coating, or to remove contaminants or other surface features from the surface of a target. These techniques may be used in diverse fields, including the aerospace industry and the food industry.

However, the hard particles typically used in such a process (e.g. sand particles) can cause unwanted effects when they are left behind. Further, if the properties of the particles, such as their hardness or size, is not carefully controlled, the amount of material removed by the blasting process may be difficult to control. Thus, there is a desire to provide apparatuses and methods for blasting which allow the properties of the particles to be controlled and to provide particles which do not cause contamination or damage to other components.

In a known arrangement, water is sprayed through a mist of liquid nitrogen inside a tank, which results in the water droplets being frozen by the liquid nitrogen to form small ice pellets. These pellets are collected in the bottom of the tank, and are then sprayed through a nozzle using compressed air in order to abrade a target surface. However, the above arrangement suffers from a number of problems. In particular, the physical properties of the pellets which are produced are not well controlled. For example, the size of the pellets produced is not well controlled, thus resulting in a large distribution of pellet sizes. This means that the pellets may melt at different rates, giving a variation in pellet hardness, which in turn may make it difficult to control the abrasion process. Likewise, the hardness of the pellets which are produced depends on the temperature at which they are produced. In particular, the hardness increases as the temperature at which the pellets are produced falls. Accordingly, as this temperature may vary, the hardness of the ice pellets may vary, which may make it difficult to control the abrasion process. Further, when the ice pellets collect at the bottom of the tank, they may coagulate, forming a mass which may block the nozzle.

It is an aim of the present disclosure to at least partially address the problems discussed above.

SUMMARY OF THE DISCLOSURE

According to an aspect of the present disclosure there is provided an apparatus for generating ice pellets, the apparatus comprising a pellet generation region, at least one nozzle configured to supply a plurality of water droplets to the pellet generation region, a liquefied gas supply configured to deliver a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets, at least one temperature measuring device configured to obtain data indicative of the temperature of the ice pellets at generation, and a control system configured to adjust the flow rate of water and/or liquefied gas to thereby control the temperature of the ice pellets at generation.

In an arrangement, a temperature measuring device may be located in or proximate to the pellet generation region.

In an arrangement, the apparatus may comprise a conduit configured to receive generated ice pellets from the pellet generation region, and a pellet gun configured to receive ice pellets from the conduit and propel them towards a target.

In an arrangement, a temperature measuring device may be located in the pellet gun.

In an arrangement, the pellet gun may comprise an unblocking device configured to dislodge ice pellets from the conduit.

In an arrangement, the apparatus may further comprise a particle filter configured to pass generated ice pellets smaller than a threshold size.

In an arrangement, the particle filter may comprise a mesh.

In an arrangement, the particle filter may comprise a rotatable disc comprising a plurality of holes.

In an arrangement, the particle filter may comprise a first rotatable disc comprising a first plurality of holes, a second rotatable disc comprising a second plurality of holes, and a distributor configured to selectively direct the ice pellets to the first rotatable disc or the second rotatable disc, wherein the average size of the first plurality of holes is smaller than the average size of the second plurality of holes.

In an arrangement, the apparatus may further comprise a pellet storage region configured to store generated ice pellets.

In an arrangement, the apparatus may further comprise an air feed configured to deliver air to the pellet storage region.

In an arrangement, the air feed may be configured to direct air to form a fluidised bed with the ice pellets.

In an arrangement, the air feed may be configured to direct air to form a cyclonic air current.

In an arrangement, the apparatus may comprise a sampling port configured to allow removal of the ice pellets.

In an arrangement, the at least one nozzle may comprise a variable aperture.

In an arrangement, the apparatus may further comprise a blade or jaws configured to mechanically deform the generated ice pellets.

According to a further aspect of the present disclosure, there is provided an apparatus for generating ice pellets, the apparatus comprising a pellet generation region, at least one nozzle configured to supply a plurality of water droplets to the pellet generation region, a liquefied gas supply configured to deliver a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets, and a particle filter configured to pass generated ice pellets smaller than a threshold size.

In an arrangement, the particle filter may comprise a mesh.

In an arrangement, the particle filter may comprise a rotatable disc comprising a plurality of holes.

In an arrangement, the particle filter may comprise a first rotatable disc comprising a first plurality of holes, a second rotatable disc comprising a second plurality of holes, and a distributor configured to selectively direct the ice pellets to the first rotatable disc or the second rotatable disc, wherein the average size of the first plurality of holes is smaller than the average size of the second plurality of holes.

In an arrangement, the apparatus may further comprise: at least one sensor configured to obtain data indicative of the particle size distribution of the ice pellets at generation; and a control system configured to adjust the flow rate of water and/or liquefied gas based on the output of the sensor.

According to a further aspect of the present disclosure, there is provided an apparatus for generating ice pellets, the apparatus comprising a pellet generation region, at least one nozzle configured to supply a plurality of water droplets to the pellet generation region, a liquefied gas supply configured to deliver a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets, at least one sensor configured to obtain data indicative of the particle size distribution of the ice pellets at generation, and a control system configured to adjust the flow rate of water and/or liquefied gas based on the output of the sensor.

In an arrangement, the liquefied gas may be liquefied nitrogen, liquefied oxygen, liquefied helium or a combination thereof.

According to a further aspect of the present disclosure, there is provided a method of generating ice pellets, the method comprising the steps of: supplying a plurality of water droplets to a pellet generation region, delivering a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets, measuring the temperature of the generated ice pellets, and adjusting the flow rate of water and/or liquefied gas to thereby control the temperature of the generated ice pellets.

According to a further aspect of the present disclosure, there is provided a method of blasting, the method comprising the steps of: generating ice pellets as described above, and propelling said ice pellets towards a target.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of non-limitative example only, with reference to the following Figures, in which:

FIG. 1 is a sectional side view of an apparatus for generating ice pellets;

FIG. 2 is a sectional side view of an apparatus for generating ice pellets including a rotatable disc as a particle filter;

FIG. 3 shows a sectional side view of an apparatus for generating ice pellets comprising a first and second rotatable disc, and a distributor; and

FIG. 4 shows a partial side view of an apparatus for generating ice pellets including an air feed.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 shows an apparatus 100 for generating ice pellets according to the present disclosure. The apparatus comprises a pellet generation region 101, and at least one nozzle 102 configured to supply a plurality of water droplets to the pellet generation region. The apparatus also includes a liquefied gas supply 103 configured to deliver liquefied gas to the pellet generation region 101 to thereby freeze the water droplets to generate a plurality of ice pellets. The apparatus 100 also includes at least one temperature measuring device 104 configured to obtain data indicative of the temperature of the ice pellets at generation, and a control system 105 configured to adjust the flow rate of water and/or liquefied gas to thereby control the temperature of the ice pellets at generation.

The apparatus 100 may include a main body 110 taking the form of a container, or hopper, as shown in FIG. 1. The main body 110 may be of any suitable shape, such as a cylindrical shape with a domed top, and a lower substantially conical region. However, it will be appreciated that the shape of the container is not limited thereto and may be of any suitable shape.

The apparatus is provided with at least one nozzle 102 which sprays water so as to form a plurality of water droplets. As shown in FIG. 1, a plurality of nozzles 102 may be provided, which results in a plurality of individual streams of water droplets being generated. The nozzles 102 may be grouped together in a central boss in a manner similar to a shower head, or may be separately mounted in the apparatus. The nozzles 102 are provided near the top of the apparatus when in use, which means that the water droplets fall under the influence of gravity towards the lower part of the apparatus. The nozzle or nozzles may have a variable aperture, which may allow for control of the stream of water droplets, namely of the flow rate and size of the droplets. The nozzle or nozzles may also be removable from the apparatus, in order to allow different types or arrangements of nozzles to be installed in the apparatus, or to allow inspection or cleaning of the nozzles or the inside of the apparatus.

The apparatus 100 is provided with a liquefied gas supply 103, which allows a liquefied gas, such as liquid nitrogen, to be supplied to the apparatus. The liquefied gas supply 103 may take the form of a pipe running around the periphery of the container, with outlets in the pipe allowing liquid nitrogen to escape from the pipe into the container. The pipe may be mounted to the container or apparatus using a plurality of brackets (not pictured). The area around the liquefied gas supply 103 forms a pellet generation region 101, in which the liquefied gas interacts with the water droplets to freeze them, thus turning the water droplets into ice pellets.

Although the above example is described using liquid nitrogen, it will be readily understood that any other suitable liquefied gas may be used as long as its temperature when liquid is sufficiently low in order to freeze the water droplets. In other words, any gas which has a temperature lower than the freezing point of water when liquefied may be used. Examples of other suitable liquefied gases include, but are not limited to, liquid oxygen and liquid helium. Further, the liquefied gas may comprise a mixture of elements such as atmospheric gases, and may be, for example, liquefied air.

The liquefied gas supply 103 may comprise a number of outlets distributed around the edge of the container thus spraying liquefied gas in a generally inward direction. Thus this may provide, as the pellet generation region 101, a layer, or curtain, of liquefied gas through which the water droplets pass and are thereby frozen. Although the liquefied gas supply is shown in FIG. 1 as being a pipe with a number of outlets, it will be understood that any other suitable means of supplying liquefied gas to the region through which the water droplets pass may be used. For example, a lattice array, spiral array or curved array may be used as the liquefied gas supply.

Thus, when the apparatus 100 is in operation, the water droplets are generated by the nozzles 102, fall through the pellet generation region 101, and are frozen by the liquefied gas supplied by the liquefied gas supply 103 to form ice pellets.

The apparatus is further provided with at least one temperature measuring device 104 in or proximate to the pellet generation region 101, and is configured to obtain data indicative of the temperature of the ice pellets at generation. Any suitable device for measuring the temperature may be used, such as an infrared camera or a thermocouple. The device may be positioned just below the pellet generation 101 such that it measures the temperature of the ice pellets which have just been generated in the pellet generation region.

The apparatus further comprises a control system 105, which is configured to adjust the operating parameters of the device in order to control the temperature of the ice pellets at generation. The control system is in communication with the temperature measuring device 104, which allows the output of the temperature measuring device 104 to be input to the control system 105, thus allowing adjustments by the control system based on the output of the temperature measuring device. The operating parameters which may be adjusted by the control system 105 include the flow rate of water, and/or the flow rate of liquefied gas. Thus, the change in temperature caused by the control system can be measured by the temperature measuring device 104, and the output fed back to the control system 105 in order to form a feedback loop, which may provide control of the temperature of the ice pellets at generation. The control system 105 may also be configured to control a variable aperture of the nozzle or nozzles. By controlling the temperature of the ice pellets at generation, their hardness can be controlled. This may allow better control of a blasting process, when the ice pellets are used in a blasting process.

The apparatus 100 may further comprise a conduit 106 which is configured to receive the generated ice pellets from the pellet generation region. This may be of particular use when the apparatus is used to generate ice pellets which are subsequently used in a blasting process. Such a conduit may be provided at the bottom of the apparatus or container when in use, such that the generated ice pellets fall under the influence of gravity to enter the conduit 106. The conduit may be, for example, a flexible pipe, and may further be thermally insulated in order to avoid melting of the ice particles during their transit through the conduit 106.

At the end of the conduit 106, there is provided a pellet gun 107 which is configured to receive ice pellets from the conduit 106, and propel them towards a target. Such a configuration may be used in order to carry out a blasting process. The pellet gun 107 may use compressed air or suction in order to propel the ice pellets. The pellet gun 107 may be mounted to a device allowing manipulation of the pellet gun 107 around six degrees of freedom (i.e. rotational and translational movement), such as a robotic arm, or a guide tube which can be inserted into a small space. This may allow the direction of the blasting process to be very finely controlled.

The pellet gun 107 may further comprise a temperature measuring device 108. This temperature measuring device 108 in the pellet gun 107 may be provided in addition to, or instead of, the temperature measuring device 104 which is located in or proximate to the pellet generation region 101. The temperature measuring device 108 located in the pellet gun 107 may allow the temperature of the generated ice pellets to be monitored. For example, the temperature of the ice pellets in the pellet gun 107 may permit indirect monitoring of the temperature of the pellets at generation. The information obtained from this monitoring may then be fed into the control system, which in turn may adjust the flow rate of water and/or liquefied gas to control the properties of the ice pellets.

In an arrangement, the temperature measuring device 108 located in the pellet gun 107 may allow a check to be made that the properties of the ice pellets have not changed substantially between being generated and being transported to the pellet gun, or may allow the properties of the generated pellets to be modified in order to anticipate changes in their properties which occur between generation and expulsion from the pellet gun 107. It will also be understood that the temperature measuring device 108 may be provided in the conduit 106 instead of in the pellet gun 107, or may be provided where the pellet gun 107 is joined to the conduit 106.

The pellet gun 107 may further comprise an unblocking device which is configured to dislodge ice pellets from the conduit 106 or from inside the pellet gun 107 itself. The unblocking device may use a mechanical or pneumatic system to dislodge ice pellets from the conduit when needed. For example, if the ice pellets increase in temperature in the conduit, they may coagulate thus blocking the conduit 106. Therefore, the unblocking device allows such blockages to be dislodged and a steady stream of ice particles to be provided.

The apparatus 100 may further comprise a particle filter 109, configured to pass generated ice pellets which are smaller than a threshold size. In other words, the particle 109 allows ice pellets which are smaller than a certain size to pass through, and blocks ice pellets which are larger than a threshold size from passing through. This may provide a further control over the size of the ice pellets, which may in turn provide for more accurate control of a blasting process. Various suitable arrangements of particle filter 109 are described below, but it will be appreciated that any suitable filter may be used. The threshold size may be chosen according to the desired function of ice pellets, and the particle filter 109 may be changeable in order to allow a variety of different threshold sizes to be chosen.

The particle filter 109 may be provided inside the main body 110 of the apparatus, as shown in FIG. 1, or may be provided in a separate component which is configured to receive the ice pellets from the main body of the apparatus. In either case, it may be configured to filter the pellets as they exit the pellet generation region 101 or at a location further downstream.

In an arrangement, the particle filter 109 may provide sufficient control over the properties of the pellets such that the temperature measuring device 104 and control system 105 may be omitted.

In an arrangement, and as shown in FIG. 1, the particle filter 109 may be formed of, or include, a mesh. The size of the holes in the mesh may determine the threshold size. The mesh may be located on a motorised device which moves the mesh to thereby dislodge the larger particles (i.e. ice pellets) which have not passed through the mesh and allow the smaller particles to pass through, thus avoiding blockage. A waste container may be provided, and arranged relative to the particle filter 109 such that particles which are too large to pass through the filter are directed to the waste container, and can be removed from the apparatus. This may be done by, for example, by angling the particle filter 109 in the apparatus, or making the particle filter dome shaped, such that the particles roll down the surface of the particle filter 109, and particles which are too large to pass through the filter are collected in the waste container. An air knife or cyclonic system may also be used to move the particles which are too large to pass through the filter away from the particle filter 109 and towards the waste container. Further, a mechanical system may be provided to press the pellets against the filter, and either push the pellets through the particle filter 109 or move them to the edge of the filter and towards the waste container. The mesh may be coated with a hydrophobic or similar coating in order to improve the flow of particles through the mesh.

In an arrangement, and as shown in FIG. 2, the particle filter 109 may use the principle of a powder feeder, including a rotatable disc 210 comprising a plurality of holes 211. The other components of the apparatus 100 may be the same as described above. The rotatable disc is configured to rotate about an axis X. Ice pellets may accumulate on the rotatable disc 210, and particles below a threshold size may fall through the holes 211 in the disc when the holes pass beneath the ice pellets. Again, the size of the holes 211 can be chosen in order to achieve the desired size of ice pellets. Further, the ice pellets which are too large to pass through the holes may be centrifuged to the outside of the rotatable disc so that they do not block the holes. As described above, the ice pellets which are too large may be disposed of by being directed to a waste container. It will be understood that the number and pattern of holes is not limited to that shown in FIG. 2, and that any suitable pattern and number of holes may be used. Alternatively, a single hole may be used at a suitable location on the rotatable disc 210.

In some situations, it may be desired to switch the size of particles which can pass through the particle filter 109. This may allow “on the fly” changes in the size of the particles. In an arrangement, and as shown in FIG. 3, the particle filter 109 may comprise a first rotatable disc 301 comprising a first plurality of holes 302, and a second rotatable disc 303 comprising a second plurality of holes 304. For the sake of clarity, the pellet generation region 101, nozzles 102 and liquefied gas supply 103 are not shown in FIG. 3, but they are substantially similar to those shown above.

The first rotatable disc 301 and second rotatable disc 303 may work in a similar way to the rotatable disc 210 as described above in relation to FIG. 2. As shown in FIG. 3, the first rotatable disc 301 and second rotatable disc 303 may rotate about the parallel axes Y and Z. As shown in FIG. 3, there is also provided a distributor 306 which is configured to selectively direct the generated ice pellets to the first rotatable disc 301 or to the second rotatable disc 302. In other words, the distributor 306 can direct, or distribute, the generated ice pellets such that they fall onto the first rotatable disc 301 or the second rotatable disc 302.

The average size of the first plurality of holes 302 is smaller than the average size of the second plurality of holes 304, thus allowing different sizes of particles to pass through each of the rotatable discs 301, 303. In other words, the size of the holes in the two rotatable discs are different from each other, which allows the extent or proportion of ice pellets that are allowed to pass through to be varied. It will also be appreciated that the number or pattern of holes may differ between the two rotatable discs 301, 303. Again, as described in relation to the rotatable disc of FIG. 2, particles which are too large to pass through the holes in each respective rotatable disc may be centrifuged to the outside of the disc.

In an arrangement, and as shown in FIG. 3, the distributor 306 may include a fixed part 305, and a moveable part 307. The fixed part 305 is configured to collect the ice pellets after they are generated in the pellet generation region 103, and may take the form of a funnel, or may be of any other suitable form. The moveable part 307 is configured to receive the ice pellets from the fixed part and which can move to be positioned such that it directs ice pellets to either the first rotatable disc 301 or the second rotatable disc 303. The moveable part 307 may take the form of a channel which is attached to the fixed part 306 by moveable joints 308.

As shown in FIG. 4, the apparatus 100 may further comprise a pellet storage region 401 which is configured to store generated ice pellets. In other words, before the ice pellets are directed to the conduit (or removed from the apparatus in some other way), the generated pellets may rest in the pellet storage region 401, which may be the lower part of the apparatus when in use. The remaining components of the apparatus are as described above and are omitted for clarity. When a particle filter 109 is present, the particle filter 109 may be positioned between the pellet generation region 101 and the pellet storage portion 401.

In an arrangement, an air feed 402 is provided which is configured to deliver air to the pellet storage region. This may help to prevent the generated ice pellets sticking together or coagulating, thus avoiding blockages.

In an arrangement, the air feed may be configured to direct air such that it forms a fluidised bed with the ice pellets. In other words, the air may surround the individual ice pellets and prevent them from coagulating. Alternatively or additionally, the air feed 402 may be configured to direct air to form a cyclonic air current. In other words, the air may form a swirling flow around the individual ice pellets, which keeps them separated from each other and prevents them coagulating.

It will be appreciated that the air supplied by the air feed may be cooled to the temperature of the ice pellets, or to a temperature cooler than the ice pellets, so that the air does not cause melting of the ice pellets. It will further be understood that the air feed need not supply atmospheric air, but may also supply any other gas suitable for forming a fluidised bed, cyclonic air current, or any other current of gas suitable for stopping clogging of the pellets. Further, although FIG. 4 shows a single inlet of the air supply 402, it will be appreciated that the air feed may supply air at multiple locations in, or proximate to, the storage portion 401. It will also be understood that a mechanical device, such as a stirrer, may also be provided instead of or in addition to the air supply 402 in order to keep the ice pellets separated from each other and prevent them coagulating.

In an arrangement, the apparatus 100 may also comprise a sample port, configured to allow removal of the ice pellets. Where one is present, the sample port may be provided in the pellet storage portion 401. The sample port may comprise an opening which allows removal of a sample of the ice pellets, which may be useful in testing or verifying the properties of the ice pellets. The sample port may also allow for equalisation of pressure between the inside of the apparatus and the atmosphere.

In an arrangement, the apparatus may include jaws or blades configured to mechanically deform the generated ice pellets. For example, it may be desired to increase the surface roughness of the ice pellets in order to increase their abrasive properties. This process may be implemented immediately after generation (i.e. in or proximate the pellet generation region 101) or, where one is present, in the pellet storage region 401. The blades may rotate in a similar manner to a blender, or the jaws may crush the pellets as the pellets pass therethrough.

In an arrangement, the apparatus may include at least one sensor configured to obtain data indicative of the particle size distribution of the ice pellets at generation. This data may indicate the proportion of particle sizes in certain ranges, an average particle size or any other suitable measure of particle size distribution. The output of the sensor may be fed to the control system 105, which may then adjust the flow rate of water and/or liquefied gas based on the output of the sensor. This may provide a further way of controlling the properties (e.g. temperature and particle size) of the generated particles. Like the temperature measuring device, such a sensor may be provided in or proximate to the pellet generation region 101, and/or in the pellet gun 107 (where present). In an arrangement, the sensor may provide sufficient control over the properties of the pellets such that the temperature measuring device 104, 108 and/or particle filter 109 may be omitted. The sensor may use an interferometric technique, or any other suitable method, and may further be configured to measure particle flux or velocity.

It will be understood that the invention is not limited to the embodiments above described and various modifications and improvements can be made without departing from the concepts described herein. Except when mutually exclusive, any of the features may be employed separately or in combination with any other features, and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein. 

We claim:
 1. An apparatus for generating ice pellets, the apparatus comprising: a pellet generation region; at least one nozzle configured to supply a plurality of water droplets to the pellet generation region; a liquefied gas supply configured to deliver a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets; at least one temperature measuring device configured to obtain data indicative of the temperature of the ice pellets at generation; and a control system configured to adjust the flow rate of water and/or liquefied gas to thereby control the temperature of the ice pellets at generation, wherein a temperature measuring device is located in or proximate to the pellet generation region.
 2. The apparatus according to claim 1, further comprising a conduit configured to receive generated ice pellets from the pellet generation region, and a pellet gun configured to receive ice pellets from the conduit and propel them towards a target.
 3. The apparatus according to claim 2, wherein a temperature measuring device is located in the pellet gun.
 4. The apparatus according to claim 3, wherein the pellet gun comprises an unblocking device configured to dislodge ice pellets from the conduit.
 5. The apparatus according to claim 1, further comprising a particle filter configured to pass generated ice pellets smaller than a threshold size.
 6. The apparatus according to claim 5, wherein the particle filter comprises a mesh.
 7. The apparatus according to claim 6, wherein the particle filter comprises a rotatable disc comprising a plurality of holes.
 8. The apparatus according to claim 5, wherein the particle filter comprises: a first rotatable disc comprising a first plurality of holes; a second rotatable disc comprising a second plurality of holes; and a distributor configured to selectively direct the ice pellets to the first rotatable disc or the second rotatable disc; wherein the average size of the first plurality of holes is smaller than the average size of the second plurality of holes.
 9. The apparatus according to claim 1, further comprising a pellet storage region configured to store generated ice pellets
 10. The apparatus according to claim 9, further comprising an air feed configured to deliver air to the pellet storage region.
 11. The apparatus according to claim 10, wherein the air feed is configured to direct air to form a fluidised bed with the ice pellets.
 12. The apparatus according to claim 11, wherein the air feed is configured to direct air to form a cyclonic air current.
 13. The apparatus according to claim 1, wherein the apparatus comprises a sampling port configured to allow removal of the ice pellets.
 14. The apparatus according to claim 1, wherein the at least one nozzle comprises a variable aperture.
 15. The apparatus according claim 1, further comprising a blade or jaws configured to mechanically deform the generated ice pellets.
 16. The apparatus according to claim 1, wherein the liquefied gas is liquefied nitrogen, liquefied oxygen, liquefied helium or a combination thereof.
 17. An apparatus for generating ice pellets, the apparatus comprising: a pellet generation region; at least one nozzle configured to supply a plurality of water droplets to the pellet generation region; a liquefied gas supply configured to deliver a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets; at least one sensor configured to obtain data indicative of the particle size distribution of the ice pellets at generation; and a control system configured to adjust the flow rate of water and/or liquefied gas based on the output of the sensor.
 18. The apparatus according to claim 17, wherein the liquefied gas is liquefied nitrogen, liquefied oxygen, liquefied helium or a combination thereof.
 19. A method of generating ice pellets, the method comprising the steps of: supplying a plurality of water droplets to a pellet generation region; delivering a liquefied gas to the pellet generation region to thereby freeze the water droplets to generate a plurality of ice pellets; measuring the temperature of the generated ice pellets; and adjusting the flow rate of water and/or liquefied gas to thereby control the temperature of the generated ice pellets and propelling said ice pellets towards a target. 